The present invention relates to methods for well log analysis and more particularly for the log analysis of complex lithology reservoirs based on density, neutron, acoustic and photoelectric absorption index measurements.
Complex lithology reservoir evaluation encounters many difficulties in situations when porous rocks such as sandstone, limestone and dolomite are combined with non-porous materials such as gypsum and anhydrite simultaneously. When the formation contains a considerable volume of non-porous material, the estimation of lithological composition is especially important for accurate determination of effective formation porosities. The analytical solution of mathematical models can be used in computerized log evaluation of complex reservoirs. In most cases, the formulas developed are simple enough to also be used by hand programmable calculators.
Studies have indicated that in the mathematical modelling of complex reservoirs not all problems encountered in the log interpretation of these reservoirs may have a unique solution. This is primarily due to the limitations of presently available suites of well logs. New methods and formulae as well as a discussion of the limitations and accuracy of this type of analysis will enable the well log analyst to perform better the evaluation of complex reservoirs and will also assist in determining when satisfactory solutions can be obtained from the well logging data or when addition information such as core data maybe needed.
Prior art publications (such as Poupon, Hoyle and Schmidt "Log Analyst and Formations with Complex Lithologies"; J. Petroleum Technology, August 1971, pp. 995-1005 and Raymer and Biggs "Matrix Characteristics Defined by Porosity Computations" SPWLA Fourth Annual Symposium 1963) have described comprehensive methods of correcting logs and porosities in complex reservoirs for the presence of shale. In the method of the present invention, it will be assumed that either the reservoir rocks are shale-free, or that corrections for shale have been made according to methods developed by these authors in the prior art.